1. Technical Field of the Invention
The invention relates to a semiconductor device and a manufacturing method thereof and, particularly, to a semiconductor device having a semiconductor chip bonded to a radiating plate and sealed with resin [resin-sealed package equipped with radiating plate (plastic package)] and a manufacturing method thereof.
2. Description of the Related Art
In a transistor for large power, generally, a radiating plate made of copper or the like is used in order to enhance the heat radiating property because the transistor generates a large heating value. A semiconductor device having a semiconductor chip bonded to such a radiating plate and sealed with resin, or a resin-sealed package equipped with radiating plate (plastic package) has a structure, for example, as shown in FIG. 1. FIG. 1 is a partially cutaway perspective view of a conventional resin-sealed package equipped with radiating plate.
A semiconductor device 10 shown in FIG. 1 is a resin-sealed package equipped with a radiating plate 2, which comprises a semiconductor chip 1, the radiating plate 2, a lead 3 and a sealing resin 5. The radiating plate 2 is formed of a metal plate such as copper and its alloy, which is plated with silver or the like. The lead 3 is solely formed by a lead frame. The sealing resin 5 is a thermosetting resin such as epoxy resin or the like. The semiconductor chip 1 is a transistor for large power.
The semiconductor device 10 is generally manufactured as follows. The semiconductor chip 1 is put on and bonded to the radiating plate 2, and the electrode (not shown) of the semiconductor chip 1 provided on its bonding surface side is electrically connected to the radiating plate 2. Electrodes 6 provided on the upper surface of the semiconductor chip 1 are connected to the inner lead part 3a of the lead 3 through bonding wires 7. Thereafter, these are put in a metal mold, the area provided with the semiconductor chip 1 of the radiating plate 2 and the periphery thereof are housed in the cavity of the metal mold, and the sealing resin 5 is filled therein and molded.
The semiconductor device 10 thus has a structure in which the sealing resin 5 is filled in the package, and the semiconductor chip 1, the bonding wires 7 and the inner lead part 3a are closely fitted to the sealing resin 5 and covered with the sealing resin 5. The outer lead part 3b of the lead 3 is protruded out of the package to form an external terminal.
The radiating plate 2 often comprises screw holes 4 for fixing the semiconductor device 10 to an aluminum chassis in the mounting of the semiconductor device.
A conventional ceramic package is described in reference to FIG. 2. FIG. 2 is a partially cutaway perspective view of a conventional ceramic package equipped with radiating plate.
A semiconductor device 11 shown in FIG. 2 has a ceramic package equipped with a radiating plate 2, which comprises a semiconductor chip 1, the radiating plate 2, a lead 3, a ceramic frame 8, and a ceramic cap 9.
The ceramic frame 8 is brazed, as shown in FIG. 2, to the center of one main surface of the radiating plate 2. Two leads 3 are brazed to the upper surface of the ceramic frame 8 opposing to each other. The leads 3 are brazed to the ceramic frame 8 so that one-side ends (inner lead parts 3a) of the leads 3 and the other ends (outer lead parts 3b) thereof are protruded to the inside and outside of the ceramic frame 8, respectively. The semiconductor chip 1 is put on and bonded to the area surrounded by the ceramic frame 8 of the radiating plate 2, and the electrode (not shown) of the semiconductor chip 1 provided on its bonding surface side is electrically connected to the radiating plate 2. Electrodes 6 on the upper surface of the semiconductor chip 1 are connected to the inner lead parts 3a of the leads 3 through bonding wires 7. The ceramic cap 9 is fixed to the upper end of the ceramic frame 8 to airtightly seal the semiconductor chip 1, the bonding wires 7 and the inner leads 3a. Namely, the semiconductor chip 1, the bonding wires 7 and the inner leads 3a are airtightly sealed in the hollow structure formed by the radiating plate 2, the ceramic frame 8, and the ceramic cap 9.
The conventional resin-sealed package equipped with radiating plate, however, had the following problems.
The resin-sealed package equipped with radiating plate is used for a semiconductor element with large heating value such as power MOSFET used in an analog amplifier or the like. The temperature of the chip surface is often raised by the heat generated in the high output operation of the semiconductor element to deteriorate or peel the sealing resin closely fitted to the chip surface, which causes the problem that the reliability of the semiconductor device is deteriorated by its characteristic change.
Since the semiconductor chip surface and the bonding wires are covered with the sealing resin, a parasitic capacity based on the sealing resin as dielectric layer is generated. The interference of the parasitic capacity often causes the deterioration of characteristics, for example, in a high frequency band of 1 GHz or more. Accordingly, the problem of deterioration of high frequency characteristics arises in the use for microwave.
The use of the ceramic package is free from such problems, but cannot enjoy the characteristic advantage of the resin-sealed package as described below.
Conventionally, a metal package, a ceramic package and the like have been used for the transistor for large power from the viewpoint of reliability, but it is desired to realize a package sufficiently reliable to the transistor for large power by use of a resin-sealed package (plastic package) made of inexpensive and highly productive molding resin.
The ceramic package can certainly realize high reliability and high performance, compared with the resin-sealed package. However, the ceramic package requires a high manufacturing cost including materials, compared with the resin-sealed package.
The adjustable or selectable range of thermal expansion coefficient in ceramic materials is narrow, compared with resin materials. Therefore, the materials of other members (metal, semiconductor and the like) constituting the semiconductor device which can be used is restricted in a narrow range, or the improvement in reliability by the matching of thermal expansion coefficient is difficult to attain.
Expensive materials such as tungsten copper, molybdenum copper and the like tend to correspond to the materials matched in thermal expansion coefficient to the ceramic materials, and the selectable range of materials is too narrow to select an inexpensive material. The material cost runs up also in this point.
At the present time when the resin-sealed package is becoming the mainstream wedging, makers having ceramic package techniques and facilities therefor are few and specialized, compared with markers having resin mold techniques and facilities therefor. Accordingly, if the resin-sealed package can be substituted with respect to the semiconductor device in which the ceramic package was conventionally adapted, a large-scaled reduction in cost can be expected.
It is desired to extend the application of the resin-sealed package to the use for the transistor for large power or the like, which was the weak point of the resin-sealed package, to inexpensively provide a semiconductor device such as transistor for large power to which high reliability by use of inexpensive and highly productive resin mold package techniques is required.